These two products are complimentary, and are apples to pears in comparison.  Each serves a purpose with some overlapping functionality.

 In the smallest environments say under 5 servers, BESR is perfectly fine as a backup app alone.  Backing up to disk, supporting Exchange, SQL, and Sharepoint.

As you get larger, have other applications or OS's that BESR doesnt support, have requirements for offsite tapes..  You will want BE.

On the flip side, BESR is far superior to BE when it comes to bare metal recovery and dissimilar hardware recovery..  IDR will not cut it, and is not as reliable in my experience.  Thus, I always recommend customers augment where it makes sense to use both products to improve recovery.  Thats the whole purpose right, not just to backup, but to actually recover, and to recover fast.

Making the assumption you have 10 servers all with varying roles..  What are the key critical servers that your company would be dead in the water without?  More than likely Email, and maybe a CRM app?  Well BE for your entire environment, and BESR for just those two critical servers to augment the data protection that BE is providing..

I've deployed, used and administrated both of these products for many years and have had others ask similar questions. Here's my take on the situation.

• If you have plenty of storage space to play with, BESR would provide an advantage over Backup Exec 12.5 in that you can do bare metal restores to different hardware. While Backup Exec offers an imaging module, it's not as robust as BESR.
   
• If your lacking in storage, I'm afraid this setup may not be possible due to the fact that now your storing images of entire machines, whereas, with Backup Exec 12.5, your backups are file-based so you only backup what's needed, not the entire hard drive. Furthermore, with deduplication becoming popular, more space savings is possible. Also, if you have slow network links, BESR may also be impossible due to the bottleneck those large images would pose.

So in short, if you have the storage, go with BESR. If you lack storage or network resources use BE 12.5.